Known materials are often incapable of satisfactorily balancing the many requirements necessary for use as optical devices. Indeed, the often competing criteria for successful optical polymers are numerous and include: (1) material must have a high transparency with little or no yellowing (greater than 90% transmission between 400 nanometers and 700 nanometers); (2) low cure shrinkage (less than 4% and in other embodiments, less than 2% linear shrinkage); (3) low “reflow” shrinkage also known as low post thermal shrinkage (less than 2% linear shrinkage upon temperature excursion between room temperature and up to 280° C., in another embodiment, less than 1% linear shrinkage under these conditions); (4) high fracture toughness (not brittle or crumbly >1.0 MPa); (5) high refractive index (greater than 1.47, and in another embodiment, greater than 1.49); (6) low dispersion (a relatively high “Abbe” number Vd—greater than and in another embodiment greater than 53); (7) ease of release from a mold; (8) good adhesion to substrates (typically quartz, glass or SiO2, which can be further optimized with additives); (9) low coefficient of thermal expansion (CTE of 120 ppm/° C., and in another embodiment less than 80 ppm/° C.); (10) a low change of refractive index with temperature (dn/dT of less than 100×10−6 RIU/° C.); and (11) must pass thermal shock (100 cycles of −40° C. to 85° C. over 2.5 minutes (i.e., 50° C./min)) without cracking or delamination and with less than 1% shrinkage. Perhaps the most difficult of these to meet are the requirements of low cure shrinkage, low dispersion, high refractive index, and high fracture toughness.
Epoxycyclohexyl-siloxane hybrid resins have been previously proposed by Crivello and others (see for example Crivello et al. Chemistry of Materials (2001) vol. 13, p. 1932). Their main advantage is their low cure shrinkage, and high transparency. While some resins based on 100% (epoxycyclohexyl)ethyltrimethoxysilane (ECHETMS) manufactured by Polyset, have Abbe number values or Vd near 56 or higher and refractive indexes of 1.5 or higher, the cured ECHETMS resin exhibit practically no glass transition and low fracture toughness (brittleness). This leads to cracking and breaking when films or other fabricated parts (optical devices) supported on glass substrates undergo thermal annealing (to between room temperature and 130° C.), reflow, or thermal shock. Overcoming low fracture toughness of cationic epoxycyclohexyl-based resins in general, and ECHETMS-based resins in particular, is a challenging issue that has received considerable attention in the published literature.
See Wu, et al., “Siloxane modified cycloaliphatic epoxide UV coatings,” 36 Progress in Organic Coatings (1999) 89-101, which teaches modification of cycloaliphatic epoxide/caprolactone polyol coatings with siloxane polyols. See Dworak & Soucek, “Synthesis of cycloaliphatic substituted silane monomers and polysiloxanes for photocuring,” 37 Macromolecules (2004) 9402-17, which describes photocurable materials including cycloaliphatic epoxide terminated poly(dimethylsiloxane-CO-methylhydrosiloxane and hydridefunctionalilzed poly(dicycloaliphatic siloxane-CO-cycloaliphatic hydroxiloxane). See also Soucke et al., “A new class of silicone resins for coatings,” 4 J. Coat Techn. Res. Vol. 3 (2007) 263-74.